A significant part of the world's lithium resources are found in natural brine sediments. Many of them contain large amounts of impurities, such as boron, magnesium and calcium as well as sulphate. In lithium recovery, the brine purifying techniques are currently based on selective precipitation of magnesium and calcium by means of sodium carbonate in such a way that the main part of the lithium will remain in the solution. The sodium carbonate is, however, a relatively expensive chemical and the sodium carbonate precipitation results in fairly high lithium losses (about 20%) as the lithium precipitates with alkaline earth metals.
At present, lithium is most widely used in glass and ceramic industry and in accumulator and battery solutions. The need for lithium in the battery industry increases as the development of electric cars advances. In lithium applications the lithium carbonate is an important intermediate product.
U.S. Pat. No. 5,219,550 discloses a process for producing a pure lithium carbonate. According to said publication, the starting material is a naturally occurring lithium-containing brine that is first concentrated by evaporation of water, either by heating or through solar energy, in pools. In the saline solution obtained after evaporation, magnesium and calcium are present as chlorides, as is lithium. In concentration, the presence of sulphates is to be avoided, but if they occur, they may be removed, for instance, by adding a calcium compound to precipitate the sulphate as gypsum. For removal of boron, it is advantageous that it is present as boric acid, and therefore to the saline solution may be added hydrochloric acid, for instance. Boron is separated from the saline solution by extracting it by means of a fatty alcohol. The boron-free saline solution is subjected to removal of magnesium and calcium by precipitation and solid-liquid separation. Lithium is separated from the brine by precipitating it as lithium carbonate using sodium carbonate.
U.S. Pat. No. 5,993,759 discloses a process for producing lithium carbonate from brine. The brine is first treated with a hydrochloric acid solution, for instance, whereby some of the boron reacts to a boric acid. The remaining boron is recovered from the solution by extraction. Next, magnesium and calcium are removed from the saline solution. Prior to removal of magnesium and calcium the solution is diluted such that the lithium content will be just 0.1 to 0.3% by weight, which prevents precipitation of large amounts of lithium in connection with magnesium precipitation. Magnesium is precipitated from the diluted saline solution in two stages, in the first stage using sodium carbonate and in the second stage using calcium hydroxide and sodium carbonate. In the second stage of magnesium removal also the calcium in the saline solution precipitates as calcium carbonate from the solution. After removal of impurities, the lithium is precipitated as lithium carbonate from the saline solution using sodium carbonate, a typical yield from recovery being about 80 to 90%.
In the article by Bukowsky, H. et al., “The separation of calcium and magnesium from lithium chloride by liquid-liquid extraction with di(2-ethylhexyl) phosphoric acid”, Hydrometallurgy, 28 (1992), pages 323-329, there is described separation of calcium and magnesium from a lithium chloride solution. According to the article, calcium and magnesium are well separable from a lithium chloride solution by using 0.5 M D2EHPA as extractant in a two-stage extraction. The same is also stated in DD patent publication 294284 (1991).